In recent years, as measures against global warming which is a global environmental problem, movement for promoting energy conservation is becoming vigorous. In these circumstances, from the viewpoint of energy conservation by effective utilization of heat, a vacuum insulation material excellent in heat insulation performance is required of an apparatus utilizing hot and cold heat. For example, the vacuum insulation material is formed by decompression-sealing a core member made of glass wool or the like in a cover member which is formed into a bag shape and has a gas barrier property.
In general, a heat conductivity of a heat insulation material is measured by a guarded hot plate method (GHP method) or a heat flow meter method (HFM method) shown in JIS A1412-1 or JIS A1412-2. However, in the vacuum insulation material, a void size of the core member is smaller than a mean free path of gas molecules under reduced pressure and therefore heat conduction by gas is small. Moreover, if the void size of the core member is as small as about 1 mm, heat transfer by convection can be ignored. Furthermore, heat transfer by radiation is small at room temperature. Consequently, heat conduction by the core member and heat conduction by a small amount of remaining gas are dominant in the heat transfer in the vacuum insulation member and therefore a heat conductivity of the vacuum insulation material is much smaller than those of other types of heat insulation material. Therefore, there is the problem that it takes time to measure the heat conductivity of the vacuum insulation material.
To solve this problem, there is a known method in which heat insulation performance (heat conductivity) of a vacuum insulation material is evaluated by measuring the degree of vacuum, because the heat conductivity of the vacuum insulation material depends on the internal pressure (degree of vacuum) of the vacuum insulation material. For example, in a vacuum degree measuring device in Patent Document 1, when a vacuum insulation material is set in a vacuum chamber and the pressure in the vacuum chamber is reduced, wrapping material of the vacuum insulation material inflates. This displacement of the wrapping material is sensed by a displacement sensor and the internal pressure of the chamber at this time is calculated as the degree of vacuum of the heat insulation material.
In an internal pressure measuring device in Patent Document 2, when an opening portion of a measuring chamber provided in a vacuum pad is brought in close contact with a thin film of a sealed container and the pressure of the measuring chamber is reduced, the thin film of the sealed container inflates. This deformation is detected by a detector and pressure in the measuring chamber at this time is measured as the internal pressure of the sealed container.